set default python to python3

Set the default python to python3 except for the py27 environment. We
have to set that explicitly to override the new default.

Change-Id: Ief7f053de13287bc3c1032870c255be4b880d172
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

tox.ini

@@ -4,6 +4,7 @@ distribute = False
 envlist = py35,py27,pep8
 
 [testenv]
+basepython = python3
 install_command = pip install {opts} {packages}
 deps =
   -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt}
@@ -14,14 +15,15 @@ commands =
   coverage report --show-missing
 
 [testenv:lower-constraints]
-basepython = python3
 deps =
   -c{toxinidir}/lower-constraints.txt
   -r{toxinidir}/test-requirements.txt
   -r{toxinidir}/requirements.txt
 
+[testenv:py27]
+basepython = python2.7
+
 [testenv:pep8]
-basepython = python3
 commands =
   flake8
   # Run security linter